Modern electronically controlled engines receive information pertaining to many characteristics of the engine control system, including the amount of coolant in the cooling system. Thus, many of such engines include a coolant level sensor that indicates when the coolant level is too low.
Typical coolant level sensors include two electrodes that are inserted in the coolant system into an area where coolant is normally found, e.g., the radiator. The electrodes will pass current between one another via the coolant. As long as there is enough coolant to surround the electrodes, the electrodes will pass current between one another. However, once the coolant falls below a certain level, then an open circuit will develop between the electrodes. This open circuit condition is detected by the engine control, which may display a warning message to the operator.
One problem with utilizing direct current through a liquid environment is that the electrodes will be exposed to electrolytes that may corrode the electrodes. Thus, most engine manufactures will generate an alternating current between the electrodes to prevent corrosion. Reference is now made to FIG. 1, which shows a prior art example of a H-bridge circuit that creates an alternating current. As shown, the H-bridge circuit includes four switches labeled S1-S4. Each switch may include a semiconductor transistor. As S1 and S2 are closed, electrical current flows through R.sub.L (which represents the resistance associated with the coolant level sensor) in a direction, denoted as current 1. As S3 and S4 are closed, electrical current flows through R.sub.L is the opposite direction, denoted as current 2. Although H-bridge circuits provide the desired alternate current, such a circuit is quite expensive due to the requirement of four switches. What is desired is a circuit that creates an alternating current with fewer components.
The present invention is directed to overcoming one or more of the problems as set forth above.